Renormalization of the one-dimensional pi-pi band gap on the Ge -111- 2x1 surface.

Using angle resolved laser photoemission spectroscopy we have measured the transient narrowing of the surface state band gap on the cleaved Ge(111) 2 x 1 surface. The reconstruction of this surface results in the formation of one- dimensional pi bonded chains of atoms which give rise to bonding and antibonding bands whose energy/wave vector dispersions are one-dimensional as well. Photoexcitation of this surface produces a transient pi population which, in turn, leads to an observed narrowing of the surface band gap. We have investigated this renormalization as a function of surface carrier density and laser excitation fluence.